In recent years, semiconductor devices having thin film transistors (TFTs) and thin film diodes (TFDs) formed on the same substrate, as well as electronic equipment having such a semiconductor device, are being developed (e.g., Patent Documents 1 and 2). Such a semiconductor device can be produced through forming semiconductor layers of the TFTs and TFDs by using the same crystalline semiconductor film that is formed on a substrate. The device characteristics of TFTs and TFDs that are formed on the same substrate are greatly affected by the structure of the semiconductor layers serving as their active regions.
Patent Document 1 discloses an image sensor having a photosensor portion utilizing a TFD and a driving circuit portion utilizing a TFT on the same substrate. Patent Document 1 discloses a display device which, by allowing a low concentration region (e.g., an n− or p− region) composing a photodiode in the photosensor to be larger than a high concentration region (e.g., an n+ or p+ region), provides an increased photocurrent, an improved efficiency of photoelectric conversion, and an improved S/N ratio (a current ratio of bright and dark). In the present specification, the p+ region and the n+ region may collectively be referred to as “high concentration regions”. Moreover, the p− region (or n− region) may be referred to as a low concentration region (or an intrinsic region; i region).